(1) Field of the Invention
The present invention relates to a polymeric resin film that is used as an information recording medium such as a magnetic tape or silver salt photographic film, an electric part such as a capacitor, a graphic art article or a packaging material, especially a polymeric resin film having improved surface characteristics, and a process for preparing this polymeric resin film.
(2) Description of the Related Art
A biaxially stretched polyester film is broadly used as a base film of a magnetic tape, a floppy disc, a photorecording medium or a silver salt photographic film.
In the field of recording media, an increase of the recording density is desired, and therefore, improvements of surface characteristics of a polymeric resin film as the base are strongly desired. For example, in a film type magnetic recording medium such as a high-density recording medium of a magnetic tape or magnetic disc, the loss at the time of recording or reproduction, which is due to the spacing loss between the recording medium and the magnetic head, is very serious. Therefore, it is eagerly desired that the surface is made smoother to reduce the spacing loss. Namely, it is required that the surface of the polymeric film as the base should be smooth. This also is desired in the field of photorecording media.
If the surface of a polymeric film is smoothened, the slip characteristic of the polymer film is degraded, and a serious problem arises in that running of the polymeric film becomes difficult in the polymeric film-preparing process or the recording layer-forming process. In short, the surface smoothness is contradictory to the easy slip characteristic in a polymeric film. However, in a polymeric film as a base of an information-recording medium, it is eagerly desired that both the requirements of surface smoothness and an easy slip characteristic will be simultaneously satisfied.
Furthermore, since it is desired that the durability of a polymeric film as an information-recording medium will be improved, it is required that the adhesion between the polymeric film and recording layer should be improved and the easy bondability of the polymeric film should be increased.
Various attempts have been made to satisfy these requirements. For example, incorporation of fine particles of calcium carbonate or silicon oxide into a polymer is generally adopted. However, according to this method, projections formed on the surface by the fine particles are too large. Accordingly, a method in which the particles to be incorporated are made much finer and these particles are homogeneously incorporated into a polymer is now under vigorous examination. However, presently available films having ultra-fine particles incorporated therein have an insufficient slip characteristic, although the surface smoothness is improved.
As means quite different from the method of incorporating such fine particles, Japanese Unexamined Patent Publication (Kokai) No. 59-35920 proposes a method in which an undrawn sheet is subjected to the corona discharge treatment in the open air and the sheet is then drawn to improve the slip characteristic. As the result of experiments made by us, it was found that the effect is insufficient and the requirements are not practically satisfied, and that no substantial improvement of the easy bondability can be attained. U.S. Pat. No. 3,489,117 discloses a technique of improving the slip characteristic of a polymer film by coating a polyester film with a resin comprising a water-soluble polymer and a silane coupling agent and drawing the coated polyester film to form a discontinuous covering film having a net-like shape and/or a node-broken net-like shape on the film surface. However, since the discontinuous covering film formed on the film surface does not have a uniform thickness and shape, the surface uniformity and easy slip characteristic are still insufficient. Moreover, since the discontinuous covering film and the base film are composed of different resins, the adhesion between the base film and the covering film is poor and a substantial improvement of the bondability cannot be attained.
As means for improving the bondability, there is generally known a method in which the corona discharge treatment is carried out in the open air, as disclosed in, for example, U.S. Pat. No. 3,018,189 and U.S. Pat. No. 1,252,935. According to this method, however, the improving effect is insufficient and the attained effect disappears in a short time.
U.S. Pat. No. 4,072,769 discloses a method in which a polyethylene terephthalate film is subjected to the plasma treatment in the presence of NO gas or NO.sub.2 gas to enhance the easy bondability. The initial improving effect by the plasma treatment is higher than that attained by the corona discharge treatment, but as in the case of the corona discharge treatment, the effect by the plasma treatment is degraded with the lapse of time. Moreover, this plasma treatment is costly.